1. Field of the Invention
The invention relates generally to lithographic image development.
The invention relates more specifically to an improved trilayer lithographic process for use within the semiconductor fabrication arts in conjunction with an image projection system having a narrow depth of focus.
2. Cross Reference to Related Applications
The following copending U.S. patent application is assigned to the assignee of the present application, is related to the present application and its disclosure is incorporated herein by reference:
(A) Ser. No. 07/893,702 filed Jun. 5, 1992 by Tatsuo Nakato et al. and entitled, SILYLATED PHOTORESIST LAYER AND PLANARIZING METHOD.
3. Description of the Related Art
High-resolution photolithographic imaging and development techniques are routinely used to mass produce high-density semiconductor devices and other microscopically sized devices (e.g. micro-machines) at low per-unit cost.
A type of reduction-projection camera, referred to in the semiconductor industry as a wafer-stepper, projects a pattern image onto a photosensitive layer (photoresist layer). The photosensitive layer is then developed to produce apertures in the photosensitive layer corresponding to the projected image. The developed photoresist layer is then hardened to withstand a next applied modifying agent. The modifying agent enters through the apertures of the hardened resist layer to act on exposed portions of the substrate.
Each successive generation of smaller-sized devices calls for a greater degree of resolution in the projected image and a finer degree of image replication in the development of the photosensitive layer. The call for images of ever-decreasing dimensions and greater resolution brings about lens designs of ever-increasing numerical aperture for each given wavelength.
Unfortunately, depth of focus decreases as numerical aperture increases at a given wavelength. It becomes difficult to faithfully develop and reproduce an image pattern in a photosensitive layer if the photosensitive layer is relatively thick and the depth of focus of the image projection optics is decreased to a point where it approaches, or worse yet, becomes smaller than the thickness of the photosensitive layer. This is particularly a problem when the photosensitive layer is conformably deposited on a nonplanar substrate surface.
A trilayer development process has been proposed recently by others (M. Tanigawa and H. Tabuchi of Sharp, Japan). The proposed process deposits a planarizing layer composed of a photoresist solution containing diazonapthoquinone (DNQ) or a like light-sensitive material, onto a nonplanar substrate surface. The planarizing layer is then hardened and flowed into planarity by baking. A liquid-like SOG (Spin-On Glass) layer is next spin-coated onto the planarizing layer and hardened by baking. Finally, a thin layer of liquid photoresist material is spin coated onto the SOG layer and bake-hardened to thereby create a solid photoresist layer having a thickness of one micron or less. The solid photoresist layer is then patterned using a high resolution imaging system and developed with an etchant that attacks the material of the photoresist layer. The SOG layer functions as an etch-stop.
There are a number of drawbacks to the proposed technique. Light sensitive components of the planarization layer, such as diazonapthoquinone (DNQ), tend to decompose and outgas nitrogen at the temperatures normally used for flowing the planarization layer, for hardening the SOG layer, and for hardening the topmost resist layer. The outgassed nitrogen bubbles can interfere with the planarity of the overall resist structure, create adhesion problems at the substrate/planarization layer interface and produce a variety of stress-related artifacts.
SOG (Spin-On Glass) generally includes particulate matter which adds to surface roughness and creates isotropy in the deposition and adhesion of the subsequently deposited photoresist layer. Defects and nonuniformity in the SOG layer also lead to isotropy in a subsequent photoresist etching (development) step. The photoresist layer has to be made sufficiently thick to compensate for roughness and etch-selectivity in the underlying SOG layer. This places an undesirable lower limit on the depth of focus that must be provided by the image projection system, and consequentially, an undesirable upper limit on the numerical aperture and resolution of the image projection system.
Finally, because wet chemistries are utilized in the formation of each of the planarizing layer, the SOG layer, and the photoresist layer, there is the danger that inadequate hardening will leave a volatile residue between layers. This residue can outgas during further processing and interfere undesirably with subsequent device fabrication steps.